The rate dependent efficacy of biochar for crop yield and nutrition on Podzols newly converted from boreal forests

Context: Under climate change and local and global food security pressures conversion to agricultural lands on Podzols developed under boreal forests is accelerating across the global north. After conversion Podzols, acid and sandy, are infertile requiring significant management to allow economic production. Biochar, while less useful on fertile soils, may accelerate fertility gains on such marginal lands; it is already considered or land-applied by local farmers, under minimal guidance. Objective: A multi-year experiment assessed the utility of single or split application of biochar (BC), and of BC doses, for beet yields and nutrient uptake on newly converted Podzols. Time and rate-dependent progression of yields, macro-and micro-nutrients uptake, all parameters identified of interest by northern farmers, were evaluated. Methods: Kiln-derived hardwood biochar was added at 0, 10, 20, 40 or 80 Mg BC-C ha-1 (BC0 to 80) either as single or as double application. Beet yields and elemental compositions were assessed over 4 years after biochar application. Work was done on a private farm in Labrador, Canada, under a scenario relevant to the boreal north. Results: BC0 could not sustain significant crop growth, suggesting that post-conversion mineralization of residual soil organic matter cannot support plant growth. In the first one to three years after application of biochar yields increased above the control, but statistically independent of application rates. A statistically significant positive biochar-rate to yield dependency was verifiable only in year 4. Nevertheless, biochar's positive effect on yields declined with time after application: e.g., BC10 matched BC0 yields in year 4. For the first three years, root ni-trogen (N) concentrations were inversely correlated to yields and plant N% declined with biochar rate. In year 4 plant N% was directly correlated to plant biomass. In general, partitioning of nutrients between beet leaves and roots was not discernibly affected by the rate of biochar; an exception was boron whose preferential accumu-lation in leaves was directly related to the rate of biochar, indicating a role for the biochar for its mobilization. Conclusions: Biochar had an immediate positive effect on the beet yields and affected plant stoichiometry. Initially this was independent of the biochar rate. Biochar's utility declined with time at a rate inversely related to its application rate. Implications or significance: Biochar is a recommendable fertility management tool for infertile, newly converted boreal Podzols; rates in the range of 10 Mg BC-C ha1 may be repeated annually or every few years as practically feasible. Long term utilization requires further monitoring of micronutrients availability and uptake to inform any necessary management adjustments. Fertilizer management must consider the time since biochar's addition to soil.